1. Field of the Invention
The invention relates to an image recording apparatus such as printing apparatus, facsimile apparatus, copying apparatus, or the like and its control method.
2. Related Background Art
FIG. 11 is a schematic diagram showing an example of an image recording apparatus. Sheets 41 as recording media are enclosed in a sheet cassette 40. A paper feed roller 42 is rotated, thereby feeding out the sheets 41 one by one. The sheet is conveyed to an image forming mechanical unit by resist rollers 43 arranged on a conveying path. A skew correction to correct a curve or the like of the sheet 41 is made by the resist rollers 43. After that, the sheet 41 is conveyed to a photosensitive drum 46 of the image forming mechanical unit.
A first sensor 44 and a second sensor 45 as sheet detecting means are arranged on a conveying path on an upstream side and a downstream side of the resist rollers 43, respectively, thereby detecting passage of a front edge and a rear edge of the sheet 41. When the second sensor 45 detects a front edge 41a of the sheet 41, the image forming mechanical unit including the photosensitive drum 46 is made operative by an instruction signal which is generated from a control apparatus (not shown) for controlling the image forming mechanical unit by using a detection signal of the second sensor 45 as a trigger signal. Image light is emitted from an LED head 48 using, for example, a light-emitting diode and irradiated onto the surface of the photosensitive drum 46 charged by a charging unit 47, thereby performing the exposure writing operation and forming an electrostatic latent image onto the surface of the photosensitive drum 46. Toner is adhered onto the formed electrostatic latent image and the image is developed by a developing unit 49 and visualized as a toner image. The toner image is transferred onto the sheet 41 by a transfer roller 50 at a pressure contact point P where the sheet is come into contact with the photosensitive drum 46. The sheet 41 onto which the toner image has been transferred is sent to a fixing unit 52 and ejected onto a stacker (not shown).
FIG. 12 is a time chart showing the sequence operation in the image recording apparatus shown in FIG. 11. After the front edge of the preceding sheet 41 passed through the first sensor 44, the paper feed roller 42 is rotated, so that the next sheet 41 is fed out from the sheet cassette 40 (t1). When the front edge of the sheet 41 is detected by the first sensor 44, the sheet 41 is further conveyed by a predetermined amount and the skew correction of the sheet 41 is made by the resist rollers 43 (t2). The driving of a paper feed motor is stopped and the resist rollers 43 are rotated, thereby conveying the sheet 41 (t3). When the front edge of the sheet 41 is detected by the second sensor 45, the exposure writing operation is started and the conveyance of the sheet 41 by the resist rollers 43 is stopped (t4). After the conveyance of the sheet 41 has been stopped for a predetermined time, the resist rollers 43 are rotated again, thereby conveying the sheet 41 (t5). When an image has been formed and the rear edge of the sheet 41 is detected by the first sensor 44, the next sheet 41 is fed out from the sheet cassette 40. After the sheet 41 was further conveyed by a predetermined amount by the resist rollers 43, the rollers are stopped and the exposure writing operation is stopped at predetermined timing. As for the preceding sheet 41, the image forming process and the sheet ejecting process are executed after that. A sequence for the subsequent sheet is repeated from timing t1 (t6).
As an example of such an image recording apparatus, there is an apparatus disclosed in JP-A-5-142879 which has already been proposed by the same applicant as the present invention, or the like. In this case, by forming a bending at the front edge of the sheet fed out from the sheet cassette by a nip portion of the resist roller, the sheet curve or the like is corrected, thereby making the skew correction. After that, the sheet is fed to the downstream side at proper timing. When the front edge of the sheet is detected by the sheet detecting sensor (second sensor), the exposure writing operation is started in the image forming mechanical unit. The resist roller is temporarily stopped synchronously with such an image forming process, thereby setting the sheet into the standby mode. The start position of the exposure to the photosensitive drum is decided in accordance with the timing when the front edge of the sheet reaches the transfer position where the toner image is transferred by the transfer roller. The resist rollers are rotated again at that timing, thereby conveying the sheet from the standby position to the transfer position.
The conventional image recording apparatus and the technique of the above Official Gazette shown as a specific example, however, have the following problems. In FIG. 11, the exposure on the photosensitive drum 46 is started when the front edge 41a of the sheet 41 which has been skew-corrected by the resist rollers 43 is detected by the second sensor 45. In this instance, in order to adjust a time which is required until a start line of the exposure writing operation, that is, the first line of the sheet 41 reaches the pressure contact point P as a transfer position of the transfer roller 50 and a time which is required until the first line of the sheet 41 reaches the pressure contact point P as a position to be printed, the rotation of the resist rollers 43 is temporarily stopped, thereby stopping the conveyance of the sheet 41. If the exposure is started after the front edge of the sheet 41 was detected by the second sensor 45 as mentioned above, since the exposure start becomes late, the printing process eventually takes time.